Reliability of amidolytic assays for progressive antithrombin and heparin cofactor activities on capillary blood

Amidolytic assay of antithrombin III on capillary blood can validly substitute for similar assays performed on venous blood, as an excellent correlation exists (r = 0.95). For the amidolytic assays of heparin cofactor activity, a much less satisfactory correlation is found (r = 0.81). Results are far more dispersed and a decrease is observed in late capillary samples. Using a low heparin concentration to assay heparin cofactor activity leads to surprisingly high activities for capillary blood. The same type of discrepancy is observed during the earliest stages of clotting of venous blood.